1. Field of the Invention
This invention relates to an apparatus and method for measuring the barrier properties of porous materials at various flow rates of particulates through the porous materials.
2. Description of the Related Act
Certain materials can be used for their barrier properties and the use may vary depending on how much of a barrier the material is intended to be. For example, filters are designed to restrict the passage of certain sized particles or particle-like matter. Other materials, such as packaging for sterilized articles, for example, would be designed to preclude the entrance of very small particles, such as microbial agents or any other substance that could compromise the sterility of what is enclosed in the package. Further, it may be desirable in some situations to allow a substance, water vapor, for example, to readily pass through a material that acts as a barrier to another substance.
It is therefore helpful to have some means for determining the barrier properties of materials. Methylene blue particulate penetration test, sodium chloride particulate penetration test and TSI 8130 automated filter tester are examples of systems used to test barrier performance or filter efficiency. These systems are usually based on drawing an air dispersion of particles at a fixed flow rate across a test sample. The flow rates tend to be relatively high because they are typically directed to ventilation applications where the volumes are very high and the medium to be tested is relatively porous, such as furnace filters. In these systems the rate of flow is typically set by the flow specification of the particle counter or detector. As such, one of the major limitations of these systems is the inability to separate the flow through the detector from the flow through the material being tested.
Another disadvantage of conventional testing methods, as previously noted, is that the materials are tested by drawing a dispersion of particles at a fixed flow rate. Because materials that have very high barrier properties (i.e., decreased porosity) produce very high pressure differentials, low face velocities can only be achieved by using unacceptably large sample sizes. Face velocity is the speed of the airflow through a material normalized for the sample size. A conventional test for barrier properties is presented in ASTM F1608-95 xe2x80x9cStandard Test Method for Microbial Ranking of Porous Packaging Materials (Exposure Chamber Method)xe2x80x9d. In this method, the face velocity is about 21 cm/sec.
It is especially important to have alternate methods for determining the barrier properties of materials used in medical packaging, because testing in that area presents some especially difficult problems. Specialized equipment and procedures are required because biological agents are typically used as the challenge particles (i.e., the particles that are introduced to the material to test its barrier properties). These test methods with biological agents can typically operate at low flow rates however, a long time is required to grow and manually count the bacterial colonies, which makes the procedure expensive. As such, relatively few materials have been appropriately tested. A conventional method of testing using microbial agents is described in xe2x80x9cA Discriminating Method for Measuring the Microbial Barrier Performance of Medical Packaging Papersxe2x80x9d, C. S. Sinclair and A. Tallentire, Medical Device and Diagnostic Industry 18(5) 228-241, 1996.
Therefore, a need exists for a relatively fast, relatively inexpensive apparatus and method for testing barrier properties of materials at various flow rates and without the necessity of using biological agents.
This invention includes a method for measuring barrier properties of a material comprising the steps of:
a) positioning a sample of the material in a holding means comprising a chamber wherein the material divides the chamber into a first portion and a second portion;
b) providing micrometer-sized particles;
c) generating a gas for mixing with the particles;
d) mixing the gas and the particles;
e) introducing the mixture of the particles and the gas into the first portion and wherein some percentage of the particles pass through the sample from the first portion to the second portion;
f) extracting from the first portion a small amount of the mixture that does not pass through the sample,
g) counting the aerosol particles from f) in a particle counter,
h) introducing a second gas through a predetermined-sized orifice into the second portion to sweep up particles that have passed through the sample;
i) extracting from the second portion the mixture that passed through the sample,
j) counting the aerosol particles from i) in the particle counter,
k) comparing the number of particles from g) to the number of particles from j).
This invention also includes an apparatus for measuring barrier properties of a material comprising:
a particle atomizer;
means for providing a gas,
means for mixing the particles and the gas;
means for transporting the gas and particle mixture;
a sample holder comprising a chamber for positioning the sample wherein the sample divides the chamber into a first portion and a second portion, the first portion having an inlet and at least one outlet, and the second portion having an inlet and at least one outlet
means for introducing a gas into the first portion through the first inlet
means for introducing a gas into the second portion through the second inlet, and wherein the mixture from the receiving means enters the first portion through the first inlet and a first percentage of the aerosol particles exits the first portion through the first outlet into a counting means and a second percentage of the mixture passes through the material into the second portion and exits the second portion through the second outlet into the counting means, and
means for comparing the particles form the first portion to the number of particles from the second portion.